Prosthetic or artificial joints replace a natural joint that has degenerated from disease or has been damaged by trauma. Prosthetic joints allow a recipient to maintain a quality of life not possible without replacement of the nonfunctional natural joint.
FIG. 1 shows one known type of prosthetic joint in the form of a hip joint 10 having a femoral component 12 and a ceramic hip head or ball 14. The hip head 14 has a bore 16 defining a frustro-conical or tapered inner surface 18 and the femoral component 12 includes a metal trunnion 20 defining a tapered surface 22. The trunnion 20 is adapted for friction fit engagement into the bore 16 of the hip head.
The femoral component 12 includes a first end defining a stem portion 24 implantable within the medullary canal of a femur and a second end defining a neck portion 24 and the trunnion 20. The hip head 14 cooperates with a prosthetic acetabular cup in an acetabular socket (not shown). The hip head 14 is rotatable within the acetabular cup to allow movement of the femur with respect to the pelvic structure of a patient.
The hip joint must bear the entire body weight of a recipient thereby requiring materials able to withstand significant loading. A hip head formed from a ceramic material is desirable due to the exceptional hardness of the material and the smooth surface finish which provides long wear and smooth gliding at an interface between the hip head and an acetabular cup into which the head is inserted. However, a ceramic hip head can be relatively brittle and subject to fracture.
The hip head bore 16 and trunnion 20 have precision machined load-bearing mating surfaces. Even if efforts are made to achieve the smoothest surface possible, surface imperfections remain. FIGS. 2A-2B show the surfaces 18,22 of the bore and trunnion in more detail. The bore and trunnion surfaces 18,22 each include respective asperities in the form of peaks 26,27 and valleys 28,29 and various surface undulations. It should be understood that throughout the drawings the asperities are shown in exaggeration for the purpose of illustration and that the prosthetic joint components are not drawn to scale.
Upon insertion of the trunnion 20 into the bore 16, a peak 26 in the surface 18 of the bore disposed opposite a peak 27 in the surface 22 of the trunnion can locally concentrate stress at a point 30 thereby overstressing the ceramic ball 14 and causing it to fracture. It will be appreciated that many such localized stress points can exist after mating of the trunnion 20 and the bore 16. Loading of the joint will exacerbate the localized stressing of the bore 16 and trunnion 20 interface.
It will be appreciated by one of ordinary skill in the art that the localized concentration of stress at opposing asperity peaks increases as the taper angle of the bore and trunnion decreases. The taper angle is defined as the angle formed by the bore surface and a longitudinal axis of the bore, and similarly for the trunnion. As is known in the art, smaller taper angles are desirable to provide a more secure engagement of the bore and trunnion to reduce the risk of disassembly in use and reduce the amount of hip head material required for the bore. Disadvantageously however, smaller taper angles magnify a wedge effect of the trunnion into the bore thereby increasing the likelihood of fracturing the hip head.
It will further be appreciated by an ordinary practitioner in the art that smaller diameter hip heads are desirable to reduce friction and allow greater offsets, and are well-suited for patients having a relatively modest frame. However, smaller hip heads provide less material to support loading of the joint and are thus more susceptible to fatigue fractures, thereby limiting their utility.
FIG. 3 shows one prior art solution to reduce the tendency of a ceramic hip head 50 to fracture. The configuration includes a metallic sleeve 52 having a tapered outer surface 54 corresponding to a tapered bore 56 in the hip head and a tapered inner surface 58 corresponding to a tapered surface 60 of a trunnion 62. For example, the ceramic hip head bore 56 has a taper angle of six degrees with respect to a longitudinal axis of the bore and the trunnion 62 has a taper angle of about three degrees. As one skilled in the art can appreciate, the sleeve must be manufactured to precise dimensions, tolerances, and surface finishes to complement the contour of a hip head bore on one surface and the trunnion on an opposing surface of the sleeve. Not only is such a sleeve complex and costly to manufacture, it does not eliminate local stress concentration at points where opposing asperity peaks are in proximity to one another. Furthermore, the trunnion is subject to disassembly in use as compared with a bore and trunnion each having a taper angle of three degrees, for example.